The invention relates to a joint-endoprosthesis having a shaft that is anchored in the marrow cavity of a tubular bone. The invention further relates to a method of fixing the seat of a joint-endoprosthesis shaft in the marrow cavity of a tubular bone.
Such joint-endoprostheses are known and serve, for instance, to replace shoulder, arm, hip, knee or foot joints of a human being. All those artificial joints have in common that they comprise at least one conical shaft, which is driven into the marrow cavity of the corresponding tubular bone for fixing the same when the endoprosthesis is inserted. In most cases the surface of the external covering of the shaft comprises supporting ribs bringing about a ratchet-like adherence of the shaft in the marrow cavity of the tubular bone when the shaft is driven in. By additionally feeding small soft bits of spongiosa into the indentations of the supporting ribs the primary stabilization and the ability of the bone to grow in after the insertion is supported.
It has, however, shown that the feeding of the spongiosa bits not always results in the desired stabilization of the endoprosthesis shaft. On one hand, this may be due to a non-optimal fit of the shaft in view of the rarely ideally formed bone and, on the other hand, to the metabolism process being different from patient to patient and the different osteogenesis connected therewith. Because of this, or also as a result of inadvertent movements when wearing the already inserted joint-endoprothesis, luxations of the endoprosthesis with a dislocation by several centimeters may occur. In such cases another operation is often required, which in most cases takes place in two steps.
An ultrasound device comprising a transportable operating element and an ultrasound head is known from U.S. Pat. No. 5,730,705, which can be placed upon the skin of the patient in the proximity of the region to be impinged with ultrasound. The propagation of the ultrasound waves and the formation of shear waves along the shaft of the endoprosthesis are illustrated in FIGS. 4 and 5 thereof. Thus, said known ultrasound device serves to impinge the gap between the spongiosa of the tubular bone and the surface of the external covering of the endoprosthesis shaft with ultrasound.
Reliable fixation of the joint endoprosthesis shaft in the bone is thus a primary concern. It is that problem which this invention addresses, its objective being an improved joint endoprosthesis of the type described above whereby a more stable fixation of the shaft in the bone is attainable.
Given a joint-endoprosthesis of the above-described kind said object is solved in accordance with the invention by providing an ultrasound source on or in the shaft, the radiation characteristics of which enable the ultrasound waves to reach a gap between the spongiosa of the tubular bone and the surface of the external covering of the shaft.
The object underlying the invention is moreover provided in accordance with the invention by a method of the aforementioned kind, wherein the gap between the spongiosa of the tubular bone and the surface of the external covering of the endoprosthesis shaft is impinged with ultrasound generated by an ultrasound source disposed on or in the shaft.
In view of the healing of bone fractures the invention makes use of the gained knowledge that the osteocytes required for the healing of a fracture multiply by the introduction of ultrasound waves into the fracture gap thereby resulting in a faster, better and more stable healing of the fracture gap. The background of said effect is that the ultrasound stimulates the cell walls of the mesenchyma cells to oscillate, which results in a desired proliferation. In respect of the use of ultrasound it is essential that it is introduced into the fracture gap, where it impinges and stimulates the spongiosa and the bone marrow, whereas it would be otherwise more or less reflected by the corticalis of the bone.
The connection of the joint-endoprosthesis shaft with an ultrasound source according to the invention and the special radiation characteristics thereof directed at the gap between the spongiosa and the shaft results in the desired stimulation of the cell walls during the operation of the ultrasound source and in the proliferation of the osteocytes. The ultrasound source may thereby be disposed externally on the shaft, orxe2x80x94which most likely is the preferred embodimentxe2x80x94may be part of the shaft or may at least be integrated in the same. A plurality of ultrasound sources may also be distributed over the circumference of the joint-endoprosthesis shaft, if a regular impingement of the gap between the shaft and the bone can only thereby be obtained.
Advantageous embodiments of the inventions are described in the subclaims.
Thus it is, for instance, provided that the ultrasound source is arranged such that the shaft serves as an oscillation carrier in the form of a resonance body. It is the goal of said improved embodiment to achieve a regular impingement of the gap between the shaft and the bone with ultrasound by the shaft uniformly introducing ultrasound into the gap.
Each ultrasound source is preferably part of an ultrasound module with an inherent voltage source so that, for operating the ultrasound source, no voltage has to be supplied from outside, in other words, a corresponding terminal on the patient""s body or even an operation are not necessary.
In order to avoid a large heat generation through the impingement of the gap with high-frequency ultrasound (approximately 1.5 MH) it is provided that the ultrasound module comprises a multi-vibrator by means of which the ultrasound source is pulsed, i.e. operated intermittently. A ratio of 2:8 between working time to non-working time has proved to be practicable.
It is the goal of the following improved embodiment to keep the ultrasound module ready to work for an as long as possible period of time without the requirement of a new operation and to make it externally operable by means of a remote control. For this purpose it is provided that the ultrasound module comprises a switch for switching the ultrasound source on/off, and a sensor for the remote-controlled activation of the switch by means of a signal transmitter. Said signal transmitter may work in the known fashion likewise on an ultrasound basis or on an infrared basis.
In order to make sure what the ultrasound module can be inserted in the shaft of the joint-endoprosthesis as easily as possible prior to the insertion of the joint-endoprosthesis, for example, for activating the voltage source beforehand, the ultrasound module is provided with a housing being accessible from outside by means of a cover plate in the shaft. Said cover plate can be removed by detaching some screws.
Despite a minimization of the current consumption of the ultrasound module it may be necessary to design the voltage source such that it can be charged from outside.
The improved embodiments of the method according to the invention provide a pulsing of the ultrasound such that an impingement lasting about 200 xcexcsec is followed by a break of about 800 xcexcsec.